L13 - Video

1
L13 - Video
Slides 2-10 courtesy of Tayo Akinwande Take the
graduate course, 6.973 consult Prof.
Akinwande Some modifications of these slides by
D. E. Troxel
2
How Do Displays Work?

• Electronic display is a Language Translator
  that converts Time Sequential Electrical
  Signals into spatially and temporally configured
  light signal (images) useful to the viewer.
• Translation Function carried out by two
  intertwined sub-functions
• Display element address wherein electrical
  signals are appropriately routed to the various
  display elements (similar to memory addressing)
• Display element (pixel) converts the routed
  electrical signal at its input into light of
  certain wavelength and intensity (inverse of
  image capture)

3
Emissive Displays

• Emissive Displays generate photons from
  electrical excitation of the picture element
  (pixels).
• Can generate energy by
• UV absorbed by a phosphor
• injection by a PN junction
• Electron Beam hitting a phosphor
• This energy causes excitation followed by
  excitation relaxation.
• Hole Electron recombination
• Exiton formation and annihilation
• Relaxation of excited ions or radicals in a
  plasma
• Sometimes the energy first goes to a dopant and
  then to photons, especially when changing the
  wavelength of the emitted light.
• Examples of Emissive Flat Panel Displays
• Electroluminescence (Light Emitting Diode),
• Cathodoluminescence (Cathode Ray Tube)
• Photoluminescence (PLasma Displays)

4
Light Valve Displays

• Light Valve Displays spatially and temporally
  modulate the intensity pattern of the picture
  elements (pixels)
• Displays that spatially and temporally modulate
  ambient lighting or a broad source of lighting
  and redirect it to the eye.
• The display element changes the intensity of the
  light using
• Refraction
• Reflection
• Polarization change
• Examples of Light Valve Displays
• Liquid Crystal Displays (active passive
  matrix)
• Deformable Mirror Displays
• Membrane Mirror Displays
• Electrophoretic Displays (E-Ink)

5
Cathode Ray Tube
Displays in the lab available for projects are
CRT displays. An electron beam boiled off a
metal by heat (thermionic emission) is
sequentially scanned across a phosphor screen by
magnetic deflection. The electrons are
accelerated to the screen acquiring energy and
generate light on reaching the screen
(cathodoluminescence)
CRT Display
Cathode
Phosphor Screen
Anode
Courtesy of PixTech
6
Flat Panel Displays

• Time sequential electrical signals describing an
  image need to be routed to the appropriate
  picture element (pixel).
• Typical flat panel displays are two-dimensional
  arrays of picture elements (pixels) that are
  individually addressed from the perimeter or the
  back. Methods of scanning include
• Sequential addressing (CRT)
• Row scan addressing
  (Thin-CRT, Plasma, Mirror, LCD)
• Row scanning of a matrix of pixels requires
  picture elements with non-linear Luminance
  Voltage (L-V) characteristics.
• If the L-V characteristics is linear (or is not
  non-linear enough), a non linear switch element
  is required in series with the pixel.

7
Thin-CRT
Cathode
Field Emission Device (FED) Display
Phosphors
Anode
Courtesy of PixTech
In principle similar to the CRT except that it
uses a two-dimensional array of electron sources
(field emission arrays) which are matrix
addressed allowing the vacuum package to be thin
8
Plasma Displays
Weber, SID 00 Digest, p. 402.

• Electrons are accelerated by voltage and collide
  with gasses resulting in ionization and energy
  transfer.
• Excited ions or radicals relax to give UV
  photons.
• UV photons cause hole-electron generation in
  phosphor and visible light emission.

9
Digital Mirror Device
Courtesy of Texas Instruments
Applied voltage deflects Mirror and hence direct
light
10
Liquid Crystal Displays
Liquid Crystals rotate the plane of polarization
of light when a voltage is applied across the cell
Courtesy of Silicon Graphics
11
Raster Scan

• Television and most computer displays use raster
  scan.

12
Composite Frames

• The frame is a single picture (snapshot).
• It is made up of many lines.
• Each frame has a synchronizing pulse (vertical
  sync).
• Each line has a synchronizing pulse (horizontal
  sync).
• Brightness is represented by a positive voltage.
• Horizontal and Vertical intervals both have
  blanking so that retraces are not seen
  (invisible).

13
Horizontal Synchronization

• The picture consists of white dots on a black
  screen.
• White is the highest voltage.
• Black is a low voltage.
• Sync is below the black voltage.
• Sync pulses are surrounded by the blanking
  interval
• so one doesnt see the retrace.

14
Composite Synchronization

• Horizontal sync coordinates lines.
• Vertical sync coordinates frames.
• They are similar except for the time scales and
  they are superimposed on each other. The numbers
  are for TV-like displays.
• What purpose is there for serrated sync?

15
(Conceptual) Recovery of Signals

• Composite video has picture data and both syncs.
• Picture data (video) is above the sync level.
• Simple comparators extract video and composite
  sync.
• Composite sync is fed directly to the horizontal
  oscillator.
• A low-pass filter is used to separate the
  vertical sync.
• The edges of the low-passed vertical sync are
  squared up by a Schmidt trigger.

16
Sync Separator

• A sync separator is used to recover sync from a
  composite video signal.
• GS4981 generates composite sync from video.
• It also generates separated sync signals.
• The sync separator is not easy to implement in
  an HDL as its input is an analog signal.
• However, your pixel clock must be synchronized
  with the recovered horizontal sync.
• If you do this synchronization with the pixel
  clock signal directly, then the pixel clock used
  will crawl a whole pixel time.
• It is better to use a faster clock, say 4 times
  faster, to do the synchronization and then the
  crawl will only be ¼ of a pixel time (distance).

17
Generation of TV Signal

• Assume one bit per pixel and provide for reverse
  video.
• This is a simple D/A to generate monochrome
  composite video.
• The S38 is an open collector part so the
  voltages are determined by the resistor network.
  The output resistance is 75 ohms.
• What signals should be glitch free?
• Vblank, Hblank, Vsync, Hsync, /LDSR,
  Normal/Reverse

18
Project for Bit-mapped Video
Store bit-mapped video in a RAM with pixels
packed into bytes. Half the time, the video
subsystem accesses the data to drive the TV
monitor. Half the time, the project can modify
(update) the bits in the RAM.
19
Timing of Control Signals

• Data is loaded into a shift register and shifted
  out to generate the video signal.
• CLK is at the pixel rate.
• TVC divides access to the SRAM giving half the
  time to get data to load into the shift register .

20
Horizontal Sync Timing

• We choose this display format.
• 256 pixels X 192 rows
• 10 MHz clock gt 200 nanoseconds per pixel
• 256 X 192 49,152 48K pixels 6 K bytes

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Vertical Sync Timing

• Our display format.
• 256 pixels X 192 rows
• 10 MHz clock gt 200 nanoseconds per pixel
• 256 X 192 49,152 48K pixels 6 K bytes

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Block Diagram of Sync Generator

• What signals need to be glitch free?

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hctr.v
/ Filename
hctr.v Description
Horizontal counter Author Don Troxel
Date 3/13/2004
Course 6.111
/module
hctr (clk, vactive, reset, hcnt,
n_srld, tvc, hblank, hsync, eol) input clk,
vactive, reset output n_srld, tvc, hblank,
hsync, eol output 80 hcnt wire
n_srld, tvc, eol, hactive reg hblank, hsync
reg 80 hcnt// parameter start
9'd224 parameter start 9'd000 assign
n_srld !(hcnt0 hcnt1 tvc) assign
tvc hcnt2 hactive vactive assign eol
(hcnt 9'b100111111) ? 1'b1 1'b0 assign
hactive (hcnt lt 9'b100000000) ? 1'b1 1'b0

always _at_(posedge clk, posedge reset) begin
if(reset 1) begin
hcnt lt start hblank lt 1'b0
hsync lt 1'b0 end else if
(hcnt 9'd319) // reset to 0 begin
hcnt lt start hblank lt
1'b0 end else
hcnt lt hcnt 1 if (hcnt 9'd255)
hblank lt 1'b1 else if (hcnt
9'd271) hsync lt 1'b1 else if
(hcnt 9'd295) hsync lt 1'b0
endendmodule
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Simulation of hctr.v
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Color TV Monitors in Lab

• Color displays are similar to three monochrome
  displays operated together, i.e., the colors add.
• Three binary signals yield an eight-color
  display.
• Well, one of the colors is black!
• Some monitors have an analog video input for each
  color.
• Sync is sometimes on a separate wire.
• Sometimes it is superimposed on the green signal.

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Character Displays (8 x 16 pixels)

• Characters are fixed bit patterns.
• They always have the same shape but can appear at
  different places on the screen.
• Use of characters can save video memory and make
  the manipulation of video memory contents simpler.

For a screen 256 x 192 one gets
384 characters. The screen address is used to
specify the position and part of the address of
the character ROM
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Character Displays (8 x 12 pixels)

• Row formatting is not as simple as before.
• But remapping is easily done in an HDL.

For a screen 256 x 192 one gets
512 characters. The screen address modified by
combinational logic is used to specify
the position and part of the address of
the character ROM
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Pairs of Characters

• Sometimes, pairs of characters can create the
  same motion effect as bit-mapped graphics.
• The speed of the motion depends on the update
  rate.
• These 24 characters (12 x 2) can display an arrow
  at any vertical position.